A vitreous silica crucible used for pulling a silicon single crystal by the CZ method has become larger along with enlargement of the diameter of the single crystal, and the crucible is exposed to higher temperature environment for a long time, and thus the crucible is required to have high strength. As the solution, for example, Patent Document 1 discloses a technique to enhance the crucible strength by applying a crystallization promoter on the crucible surface to crystallize the crucible surface under high temperature during the pulling process. Furthermore, Patent Document 2 discloses a technique to provide, on the outer surface of the vitreous silica crucible, a stabilization layer having a softening temperature higher than vitreous silica.
Furthermore, Patent Documents 3 and 4 disclose a vitreous silica crucible having three-layer structure which is comprised of an outer layer of an aluminium (Al)-doped silica layer, an intermediate layer of a natural silica layer or high-purity synthetic silica layer, and an inner layer of a transparent high-purity synthetic silica layer. Furthermore, Patent Document 5 discloses a vitreous silica crucible having three-layer structure on the sidewall portion and the curved portion, and having two-layer structure on the bottom portion. The three-layer structure includes an outer layer of crystallization promoter-doped vitreous silica, an opaque intermediate layer of natural vitreous silica, and a transparent inner layer of natural or synthetic vitreous silica. The two-layer structure includes an opaque outer layer of natural vitreous silica, and a transparent inner layer of natural vitreous silica or synthetic vitreous silica. According to the structure, contact between the crucible bottom portion and a susceptor can be enhanced, and thus the crucible can be stably supported.
In the above-mentioned pulling of a silicon single crystal, the vitreous silica crucible is mounted in a susceptor, and the entire crucible is heated, and thereafter the polycrystalline silicon is charged and melted inside the crucible, and a single crystal is pulled therefrom. The inner surface of the vitreous silica crucible contacting the silicon melt corrodes, and thus after single or multiple pulling, a corroded vitreous silica crucible is left in the susceptor. Some silicon melt is left in the used vitreous silica crucible, and when the silicon melt and the crucible are cooled, distortion is generated due to the difference in the thermal expansion coefficient between the crucible and the silicon melt. This distortion damages the crucible and leaves the damaged used crucible together with silicon melt in the susceptor. The damaged used crucible cannot be re-used, and thus is scrapped after taking-out of the susceptor.
In the conventional vitreous silica crucible, when the used crucible is to be taken out of the susceptor, the crucible sometimes adheres to the susceptor, and thus is difficult to be taken out of the susceptor. In such a case, it is necessary to break the portion adhering to the susceptor by use of a tool, and the handling is difficult for a large-size crucible, and the expensive susceptor is sometimes damaged. Therefore, users have a strong need of a vitreous silica crucible whose can be easily taken out of the susceptor after the pulling.
In order to solve this problem, in Patent Document 6, a crystallization promoting layer is formed so as to form a crystallized layer having a thickness of 0.5 to 2 mm by crystallizing the outer surface at high temperature. This crystallized layer at the crucible sidewall portion is finely crushed during cooling after termination of pulling, and allows easy taking-out of the crucible.